1. Field of the Invention
This invention relates to a method and apparatus for passing threadlike pieces through tubular products, and more particularly to a method and apparatus for passing threadlike pieces through tubular products by vibration.
The method and apparatus of this invention is used for making optical fiber cables, electric cables, composite conduits and the like comprising optical fibers, metal wires and other threadlike pieces passed through protective tubes or sheaths.
2. Description of the Prior Art
Threadlike pieces are often passed through long tubular products. Optical fiber cables recently coming into extensive use, for example, are often metal-covered to make up for the insufficient strength of optical fibers. In such instances, core wires or cords of optical fibers are passed through tubes not more than a few millimeters in diameter and over several hundred meters in length. Sometimes, a messenger wire of steel or other metal is passed before a core wire or cord is passed.
Typical conventional methods of making such products comprising threadlike pieces passed through metal and other tubes are disclosed in EPC Patent No. 91717 and Japanese Provisional Patent Publication No. 44010 of 1987. In these methods, carrier members or tubes (hereinafter generically called tubes) through which threadlike pieces are passed are vibrated. A threadlike piece is passed through a tube by means of the carrying force imparted thereto on the "vibrating conveyor". When a tube is as long as, for example, over tens of meters, the tube is coiled for ease of handling, and a threadlike piece is passed through the coiled tube.
A threadlike piece passed through a long tube according to such conventional methods often stops part way through. The threadlike piece once stopped remains at a standstill even if the vibration of the tube continues. On the occurrence of such a blockage, the entire length of the inserted threadlike piece or a considerably large portion thereof has had to be pulled out of the tube for starting the job all over again. As such, conventional methods of passing threadlike pieces through long tubes have been inefficient and time-consuming. The following is the cause for the stopping of a threadlike piece part of the way through a tube the inventors have found.
In the conventional methods, the inlet end of a tube vibrates integrally with a coil. The vibrating inlet end coming in contact with a threadlike piece exerts a force thereon that works in the direction of the diameter of the tube. Therefore, a portion of the threadlike piece upstream of and close to the inlet end of the tube moves vigorously in that direction. As a consequence, a centrifugal force flicks the portion of the threadlike piece near the inlet end out of the tube, thereby obstructing the further advance of the threadlike piece into the tube. Also, the rapid diametrical motion of the inlet end causes the same portion to remain ahead of the inlet end, thereby discharging the threadlike piece in the tube. Furthermore, the vibration repeatedly bends and damages the threadlike piece near the inlet end of the tube. Microcracks are a typical damage to optical fibers, in particular. The vibration-induced contact with the inlet end of the tube can produce an abrasion on the surface of the threadlike piece.
Usually, a threadlike piece paid off from a spool or bobbin is fed to a tube into which the piece is to be passed. If the pay-off (or the feed) of the threadlike piece from the spool etc. lags, for some reason, behind the travel forward by vibration, the threadlike piece is pulled backward, no longer advancing into the tube.
A threadlike piece winds forwards with undulating motion through a tube. The advance of the forward end of the threadlike piece lags behind that of the following portion or stops when the forward end trips against irregularities on the inner surface of the tube or foreign matters therein, or when it is subjected to a backward force from near the top surface of the inner wall. Pushed by the following portion, the forward end of the threadlike piece on such occasions suddenly makes heavy undulations. Consequently, peaks of such undulations in the forward end of the threadlike piece strike hard against the inner wall of the tube, offering sufficient resistance to suddenly prevent the further passing of the threadlike piece therethrough.
The part way stalling of a threadlike piece in a tube is due to any one of or a combination of the three causes just described.